Electric indicator with return-to-zero feature and compensating coil to cancel the return-to-zero feature during measurement

ABSTRACT

An air core meter movement having return-to-zero means. Permanent magnets mounted within the deflection coils provide a field which interacts with the rotor magnet to return the pointer to zero when no power is applied to the meter. A compensating coil is also included to counteract the effects of the permanent magnets when measurable signals are applied to the deflection coils.

FIELD OF THE INVENTION

This invention relates generally to electrical indicators, and moreparticularly to an air core meter movement having a means for returningthe pointer to the zero position when power is disconnected from themeter movement.

BACKGROUND OF THE INVENTION

In electrical indicators, often classified as analog devices such asvolt meters, ammeters, and other dial and pointer-type instruments, ithas been necessary to address the problems associated with returning thepointer to zero when no power is applied to the meter. Over the courseof many years attempts have been made to provide this function by avariety of means, including electrical, mechanical, magnetic, orcombinations thereof. In some types of meter movements, it is relativelyeasy to provide means to return the pointer to zero. However, in aircore or vector movement meters where springs are not used, when theelectrical power is turned off, the pointer does not automaticallyreturn to zero, but it will normally remain in the position it was whenthe power was turned off. On the other hand, the pointer may come torest at some arbitrary position unrelated to the reading at the time thepower is disconnected. In some instances, the fact that the meter doesnot return to zero is a cosmetic problem and more a matter of choicethan a matter of necessity. However, in some applications there could bea direct relationship between safety and the reading of a meter with nopower applied. For example, in aircraft, if there is a loss ofelectrical power to an instrument, the fact that it does not return tozero can result in the operator's perceiving a false reading and therebyreacting improperly to a situation where he does not have accurateinformation.

Air core meter movements have replaced the D'Arsonval movements, whichwere the standard for analog meters for many years, in many automotiveuses. This is because the air core movement is more rugged than theD'Arsonval movement, the air core movement has no springs, and thetorque applied to it is high enough so that the bearing arrangement maybe formed as a rugged throughshaft system as opposed to employingjeweled bearings. An air core instrument employing coils which generateorthogonal magnetic fields for purposes of deflecting the movement rotoris shown in U.S. Pat. No. 3,168,689. Another aspect of air core metersis shown in U.S. Pat. No. 3,460,038. These patents describe thestructure and operation of a basic air core meter, the type of meterwhich is the subject of the present invention.

Examples of patents which disclose particular means for providing areturn-to-zero function, and patents which disclose the use of auxiliarypermanent magnets within the meter movement for some purpose are listedbelow.

U.S. Pat. No. 3,777,265 employs magnets for the zero or restoring force,and an external electrical current is employed to provide the field fordeflecting the meter pointer.

U.S. Pat. No. 3,995,214 employs magnets for biasing and compensatingonly for purposes of improving the linearity of the meter response toinput signals.

Another example of a patent which employs a stationary permanent magnetto provide return torque is U.S. Pat. No. 4,090,131. In this patent, themagnetic field of the permanent magnet interacts with the rotor flux toprovide a return torque which varies linearly with the rotor angle.

Other patents which disclose the use of permanent magnets for zeroingthe meter movement are U.S. Pat. Nos. 2,668,945 and 3,094,659.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a simplereturn-to-zero means in an air core meter. It is another object toprovide the return-to-zero function together with compensation meanswhich obviates the effects of the return-to-zero means when a signal tobe indicated is applied to the meter. With this compensation means, theinfluence of the return-to-zero function applies to the meter rotor onlywhen power is removed from the instrument.

Broadly speaking, this invention is concerned with an air core meterhaving the normal orthogonally positioned deflection coils to positionthe rotor in response to input signals which are proportional to aquantity being measured. That quantity may be primarily voltage orcurrent, but it is normally directly proportional to various physicalphenomena such as level, speed, temperature, and the like. Morespecifically, this invention employs two permanent magnets located oneither side of the bobbin or frame in which the rotor is mounted,thereby providing a field which, when no other deflection forces areapplied to the rotor, returns the rotor to a preset or zero position.Wound directly about the bobbin is a compensating coil which applies anoppositely directed field to the rotor to cancel the effects of thefield provided by the permanent magnets. This compensating coil operateswhenever power is applied to the meter, whether or not a measurablesignal is also applied simultaneously. The orthogonally wound deflectioncoils, commonly called sine and cosine coils, are then wound about thebobbin in a known manner.

A major advantage of the present invention is that the return-to-zerofunction, together with a means to cancel that function during normalindicating operation of the meter, are added in a very simple manner toa conventional air core meter movement without in any way affecting thenormal indicating function of the meter. An additional advantage is thatthe return-to-zero device and the compensation means may be designed sothat changes in the magnetic field strength of the permanentreturn-to-zero magnets resulting from ambient temperature changes arebalanced by changes in thermal conductivity of the compensation coilwindings.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects, advantages, and features of this invention will be moreclearly understood from the following detailed description when readtogether with the accompanying drawing, in which:

FIG. 1 is a front view of a meter in the form of a tachometer in whichthe present invention is incorporated;

FIG. 2 is a perspective view of the movement of an air core meter of thetype employed in this invention;

FIG. 3 is a perspective view of the bobbin used in the movement of FIG.2 showing the location of the return-to-zero magnets;

FIG. 4 is a view similar to FIG. 3 showing the compensating coil woundon the bobbin;

FIG. 5 is a simplified schematic diagram of the meter circuit of theinvention including the compensating coil;

FIG. 6 is a vector diagram showing the effect of the return-to-zeromagnets when the power to the meter is off;

FIG. 7 is a vector diagram showing the effects of applying power and asignal to the meter; and

FIG. 8 is a vector diagram similar to FIG. 7 showing the effect of adifferent signal applied to the meter.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference now to the drawing and more particularly to FIG. 1thereof, there is shown a meter, in the form of a tachometer, generallyreferred to by the reference numeral 11. Within the bezel 12 is normallya protective glass 13 beneath which is indicating face 14. A shaft 15projects through the center of the indicating face, and pointer 16 ismounted thereon. In the position shown in FIG. 1, pointer 16 is alignedwith the "0" indication. Furthermore, the meter of FIG. 1 is shown as a270° meter. This invention functions effectively as part of any meterfor which an air core movement is applicable. Such meters normally havemovements ranging between 90° and 270°, but air core movements may bemade which have larger or smaller arcuate scales.

A completed meter movement 20, before mounting in the meter case, isshown in FIG. 2. The main supporting frame of the meter movement isbobbin 21 which includes a housing cavity 22 within which is rotatablymounted rotor 23. The rotor is preferably made of a permanent magneticmaterial and is magnetized with distinct diametric or radially oppositenorth and south magnetic poles. An example of such material which can beemployed in an air core meter is referred to by the name "Lodex" (atrade name of General Electric Company). The housing provides a sealedenvironment for the rotor. It is normally desirable to provide some typeof damping means for the rotor. A damping fluid such as a silicone maybe put into cavity 22, or a more viscous substance such as grease may beused in the meter bearings in such a manner as to surround the rotorshaft at either or both ends. The shaft 15 is mounted to the rotor, andthe bobbin provides journal bearings for either end of the shaft inknown manner. Pointer 16 is mounted to one end of shaft 15. In finalassembly, of course, the dial face 14 would be positioned on the metermovement beneath pointer 16. Wound about bobbin 21 is inner or cosinecoil 25 and outer or sine coil 26. Note that these deflection coils areorthogonally wound around the bobbin. In this manner, coils 25, 26provide orthogonal electromagnetic fields. The fluxes produced by thedeflection coils are mutually perpendicular and intersect at a point inthe center of the windings. The structure of the meter movement is suchthat this intersection point is on the movement (and shaft) axis and atthe center of rotor 23. Additionally, the axes of the fluxes lie in theplane of the center of the rotor 23. The basic functioning of the metermovement disclosed in FIG. 2 is adequately described in previouslymentioned U.S. Pat. No. 3,168,689 and is well known in the art.Therefore, a detailed description of the meter movement itself will notbe set forth herein.

FIG. 3 shows the bobbin of the meter movement at a preliminary stage ofassembly. The bobbin itself is made of a relatively inexpensive,non-conductive and non-magnetic material such as an appropriately rigidplastic. The bobbin is formed with corner posts 31, 32, 33, and 34 andhousing section 35 which interconnects the four corner posts andprovides the housing cavity 22 discussed previously. Small recesses 36are formed in housing 35 on diametrically opposite sides thereof, in ahorizontal plane which passes through the center of the housing cavityand consequently through rotor 23, as viewed in FIG. 3. In theserecesses are secured small discs 37 which are each a permanentlymagnetized ferrite. A material which is satisfactory for these magnetsis sold under the tradename "Plastiform," owned by Leyman Corporation.These ferrite discs, when subjected to an appropriate magnet field,become strong permanent magnets and as mounted form the return-to-zeromagnets of the meter of the present invention. The return-to-zeropermanent magnets may be of any shape or form, and are arranged, as forexample NS--NS or SN--SN (as viewed on a line through these magnetswhich intersects the axis of rotor 23) to establish the restoringreturn-to-zero magnetic field. It is only necessary that they functiontogether with the magnetic poles of the rotor to urge the rotor to apreset position. These magnets may be shaped other than as discs, theymay be small groupings of magnetic elements instead of single magnets37, or they may even be formed integrally with the bobbin.

A compensating coil 41 is then wound around housing 35 (FIG. 4) in amanner such that, when current is passed therethrough, the fieldproduced by the coil will be on the same axis as that produced bymagnets 37, but in the opposite direction. The ends of the compensatingcoil wire are connected to terminals 42 and 43 on respective cornerposts 33 and 34. For reference purposes, the wires of the compensatingcoil are wound on the bobbin with the same physical orientation as thecosine deflecting coil. The cosine coil comprises an appropriatemultiplicity of turns of copper wire wound over the top of thecompensating coil, the ends of the cosine coil being connected toterminals 40 and 43 on respective corner posts 31 and 34. The sine coil,also comprising an appropriate multiplicity of turns of copper wire, isthen wound on the bobbin, the turns being physically orientedorthogonally with respect to the cosine coil. Because one coil is closerto the rotor than the other, the number of turns of the sine coil maydiffer somewhat from the number of turns of the cosine coil. The sinecoil wire is connected to terminals 39 and 43 on respective corner posts32 and 34. The completed device is shown in FIG. 2, where permanentmagnets 37 are enclosed within the sine and cosine coils and are spacedon housing 35 by 180°. These magnets are oriented physically on thecenter line of the sine coil winding, that is, the turns of wire of thesine coil cover magnets 37, whereas the cosine coil turns left themagnets exposed on the sides of the housing.

The meter movement of this invention is shown and described as havingthe coils wound on the bobbin in the order of compensating, cosine, andsine coils. However, it is quite feasible to wind the coils in anyorder. It is also possible that the sine coil could be wound inalignment with the compensating coil instead of orthogonally withrespect to it. Such variations in structure do not affect the functionof the invention.

FIG. 5 schematically shows the operating circuitry of the meter of thisinvention. A function generator 45 may be any device which provides thesignals to the coils which are indicative of a phenomenon to bemeasured. As indicated previously, this could very well be speed orrevolutions per minute, and the instrument would then be a tachometer.Cosine coil 25 and sine coil 26 are connected to signal source 45 andhave a common terminal 43. Terminal 43 provides a common or referencevoltage in the instrument circuit. The object of Zener diode 47 is tomaintain, together with resistor 55, a substantially constant voltage oncommon terminal 43, that is, a reference voltage. Compensating coil 41is connected between reference voltage terminal 43 and ground throughresistor 51. DC voltage source 52, which may be the battery of anautomobile, is connected to the compensating coil through switch 53 andresistor 55. Thus, it is immediately evident that when switch 53, whichmay be the ignition switch of the automobile, is closed, a bias voltageis applied to compensating coil 41 for purposes to be further describedbelow, and to common terminal 43 to provide the circuit referencevoltage. Function generally 45 may be an integrated circuit (IC) deviceand may have several functions normal in such an instrument which do notrelate to the present invention. Therefore, the function generator willnot be described in detail beyond stating that it provides the necessarysignals in a known manner to cosine and sine coils 25, 26, pursuant tosignals applied thereto by means of external signal source 56. A voltageof sufficient magnitude is readily available in an automobile to providethe power necessary for operating the instrument represented by FIG. 5.

The effect of the return-to-zero magnets and the meter coils will now beexplored in greater detail with reference to FIGS. 6-8, which are vectordiagrams representing the various forces which may be applied to themeter movement rotor. When the meter is in the quiescent condition, thatis, when switch 53 is open and no power is applied to the meter, FIG. 6is applicable. As stated previously, return-to-zero magnets 37 have aflux or magnetic field axis such that the rotor is urged to a positionof rest, which may be defined as the zero position of pointer 16. Theflux of magnets 37 is represented by vector Q_(Z) on the zero axis inFIG. 6. No other magnetic or electromagnetic fluxes are applied to therotor when switch 53 is open.

In FIG. 7 it is assumed that switch 53 is closed and that a signal issupplied to the deflection coils by means of function generator 45pursuant to a signal from external input 56. When switch 53 is closed,compensating coil 41 provides a field or flux Q_(C) which is equal andopposite in direction to field or flux Q_(Z) produced by magnets 37.Thus, it may be seen that as soon as the switch is closed and power fromvoltage source 52 is applied to the meter movement, the effect ofreturn-to-zero magnets 37 is removed from the meter rotor. In FIG. 7 asignal is applied to cosine coil 25 of magnitude X₁ and to sine coil 26of magnitude Y₁, thereby having a resultant flux direction and magnitudeindicated by vector Q_(R). This would indicate that pointer 16 wouldread upscale by the same number of degrees from the zero position asvector Q_(R) is rotated from the zero axis.

FIG. 8 indicates a different position of the resultant flux vector Q_(R)pursuant to cosine signal amplitude -X₂ and sine coil of magnitude Y₂.Note that vectors Q_(Z) and Q_(C) are equal and opposite in direction,as they were in FIG. 7, so that again, return-to-zero magnets 37 andcompensating coil 41 exactly offset each other and do not in any wayaffect the position of rotor 23.

By choosing appropriate materials for various elements of the metermovement, the return-to-zero function of the meter may also betemperature compensated. The strength of ferrite magnets 37 is notstable with temperature. The strength of the magnetic field provided bythese magnets decreases substantially linearly with increase intemperature. However, as temperature increases, the resistance of copperwire also increases. Thus, as the magnetic strength of the permanentmagnets goes down, the current through compensating coil 41 alsodecreases substantially linearly. It would normally be expected that therate of change of magnetic strength of magnets 37 would differ somewhatfrom the rate of change of resistance in the copper wire. For thisreason, resistor 51 is connected in series with compensating coil 41,the ohmic value of which is chosen so that the effect of thecompensating coil substantially balances the effect of thereturn-to-zero magnets over a relatively wide range of temperatures inwhich the meter might be required to operate. The resistance of resistor51 is substantially independent of temperature changes.

In view of the above description, it is likely that modifications andimprovements will occur to those skilled in this art which are withinthe scope of this invention.

What is claimed is: .[.1. An electrical indicator having an indicatorpointer and means for angularly displacing said pointer in response tovariations in the magnitude of an external electrical signal, saidindicator comprising: said orthogonal second and third coils..]. .[.4.An electrical indicator as in claim 3, wherein:said permanent magnetmeans for establishing said first magnetic flux comprises a pair ofpermanent magnets disposed in the plane of rotation of said rotor onopposite sides of the rotational axis thereof..]. .[.5. An electricalindicator as in claim 4, wherein: said first coil is disposed inalignment with one of said second and third orthogonal coils..]. .[.6.An electrical indicator as in claim 5, which further includes: a stableDC voltage source for energizing said first coil with said predeterminedcurrent for establishing said second magnetic flux for substantiallycancelling said first magnetic flux..]. .[.7. An electrical indicator asin claim 6 which further includes: a function generator adapted toconvert said external signal to a pair of signals for application,respectively, to said second and third orthogonal coils..]. .[.8. Anelectrical indicator for angularly displacing an indicator pointer inresponse to variations in the magnitude of an external electrical signalsource, said indicator comprising: a bobbin formed with a centralhousing having a cavity therein; a rotor within said cavity, said rotorbeing radially magnetized to provide opposite magnetic poles; a shaftmounted to the axis of said rotor and extending outwardly from saidcavity; means for rotatably mounting said rotor and said shaft to saidbobbin; permanent magnet means on either side of said housing, arrangeddiametrically with respect to said rotor, the axis of the flux resultingfrom said magnets passing through said rotor perpendicular to the axisthereof and oriented in a first predetermined direction and urging saidrotor to an equilibrium position; a first coil on said bobbin comprisinga multiplicity of turns of conductive wire, said first coil beingoriented so that, when current passes therethrough, a flux is createdthrough said rotor having an axis in alignment with the axis of the fluxproduced by said permanent magnets; a second coil on said bobbincomprising a multiplicity of turns of conductive wire, said second coilbeing oriented so that, when current passes therethrough, a flux iscreated through said rotor having an axis displaced 90° from the axis ofthe flux produced by said first coil; means adapted for connecting saidfirst and second coils to a DC voltage source; means adapted forcoupling said first and second coils to a source of external signals;and a third coil on said bobbin comprising a multiplicity of turns ofconductive wire, said third coil being adapted to be coupled to said DCvoltage source and being oriented in alignment with said first coil, theaxis of the flux produced by said third coil being in alignment with andopposite in direction to the flux produced by said permanent magnets;wherein when said DC voltage source is selectably connected to saidthird coil, a flux is produced thereby which is opposite to said firstpredetermined direction and equal in magnitude to the flux produced bysaid permanent magnets; and wherein said pointer is selectively movedfrom said equilibrium position pursuant to the resultant flux when asignal is applied to said first and second coils and said pointerreturns to said equilibrium position responsive to the flux of saidpermanent magnets when there is no signal on said first and second coilsand no DC voltage source connected to said third coil..]. .[.9. Theelectrical indicator set forth in claim 8 wherein said permanent magnetmeans comprises two discrete magnets on either side of said housing onthe center line of said rotor..].
 10. .[.The electrical indicator setforth in claim 9.]. .Iadd.An electrical indicator for angularlydisplacing an indicator pointer in response to variations in themagnitude of an external electrical signal source, said indicatorcomprising:a bobbin formed with a central housing having a cavitytherein; a rotor within said cavity, said rotor being radiallymagnetized to provide opposite magnetic poles; a shaft mounted to theaxis of said rotor and extending outwardly from said cavity; means forrotatably mounting said rotor and said shaft to said bobbin; permanentmagnet means disposed radially outward from the axis of said shaft,arranged diametrically with respect to said rotor, the axis of the fluxresulting from said magnets passing through said rotor perpendicular tothe axis thereof and oriented in a first predetermined direction andurging said rotor to an equilibrium position; a first coil on saidbobbin disposed to intersect the axis of said shaft comprising amultiplicity of turns of conductive wire, said first coil being orientedso that, when current passes therethrough, a flux is created throughsaid rotor having an axis in alignment with the axis of the fluxproduced by said permanent magnet means; a second coil on said bobbindisposed to intersect the axis of said shaft comprising a multiplicityof turns of conductive wire, said second coil being oriented so that,when current passes therethrough, a flux is created through said rotorhaving an axis displaced 90 from the axis of the flux produced by saidfirst coil; means adapted for connecting said first and second coils toa DC voltage source; means adapted for coupling said first and secondcoils to a source of external signals; and a third coil on said bobbindisposed to intersect the axis of said shaft and comprising amultiplicity of turns of conductive wire, said third coil being adaptedto be coupled to said DC voltage source and being oriented in alignmentwith said first coil to include the rotor therein, the axis of the fluxproduced by said third coil being in alignment with and opposite indirection to the flux produced by said permanent magnet means, whereinwhen said DC voltage source is selectively connected to said third coil,a flux is produced thereby which is opposite to said first predetermineddirection and equal in magnitude to the flux produced by said permanentmagnets and independent of the currents flowing through said first andsecond coils, wherein said pointer is selectively moved from saidequilibrium position pursuant to the resultant flux when a signal isapplied to said first and second coils and said pointer returns to saidequilibrium position responsive to the flux of said permanent magnetswhen there is no signal on said first and second coils and no DC voltagesource connected to said third coil, wherein said permanent magnet meanscomprises two discrete magnets on either side of said housing on thecenter line of said rotor, and .Iaddend.wherein.[.:.]. said third coilis comprised of turns of wire having an impedance which varies directlywith temperature; the magnetic field strength of said permanent magnetmeans varies inversely with temperature; a resistor connected in serieswith said third coil; and whereby the combined impedance of said thirdcoil and said resistor varies with temperature such that the flux ofsaid third coil substantially nulls the effect on said rotor of the fluxof said permanent magnet means substantially independently oftemperature.
 11. The electrical indicator as in claim .[.9.]. .Iadd.10.Iaddend.wherein said coupling means comprises a function generatoradapted to be connected to a source of external signals, said functiongenerator converting said external signals into appropriate signalscoupled to said first and second coils for deflection of said rotor fromsaid equilibrium position. .[.12. An electrical indicator having apointer and means for angularly displacing said pointer in response tovariations in the magnitude of an external electrical signal, saidindicator comprising:a rotor retaining said indicator pointer and beingmagnetized to provide radially opposite magnetic poles; means forrotatably mounting said rotor and said indicator pointer; permanentmagnet means for establishing a first magnetic flux passing through saidrotor substantially perpendicular to the axis thereof and urging saidrotor to an equilibrium position; means for compensating the first fluxgenerated by said permanent magnet means, wherein the urging to anequilibrium position of said rotor by said first magnetic flux iscancelled; a first and a second coil means responsive to said externalelectrical signal and oriented to establish a rotatable flux disposed atan angle representative of said external signal magnitude, wherein:saidmeans for compensating is selectively operative to alter the fluxcreated along the axis of at least one of said first and second coilmeans; said rotor and pointer rotate from said equilibrium position to aposition at said angle of said rotatable flux when at least one of saidfirst and second coil means are energized as aforesaid; and wherein saidrotor and said pointer return to said equilibrium position establishedby said first flux of said permanent magnet means when said means forcompensating is inoperative and said first and second coil means are notenergized..]. .[.13. The electrical indicator of claim 12, whereinsaidmeans for compensating comprises means to cancel the flux generated bysaid permanent magnet means..]. .[.14. A method of angularly displacinga pointer of an electrical indicator in response to the magnitude of anexternal electrical signal, comprising the steps of: rotatably mountinga magnetized rotor having radially opposed magnetic poles; establishinga first magnetic flux passing through said rotor to engage saidmagnetized rotor to urge said rotor to an equilibrium position;generating a rotatable flux oriented to an angle corresponding to themagnitude of said extend electrical signal; compensating for the firstmagnetic flux when said rotatable flux is generated, wherein:said rotorand said pointer rotate from said equilibrium position to coincide withsaid angle of said rotatable flux, and said step of compensating isoperative while generating said rotatable flux, and inoperative whensaid rotatable flux is not generated..].